This application relates to an electrical generation system and method where a constant frequency generator is associated with a gas turbine engine and is capable of operating in both a start mode (as a starter) and a generate mode (as a generator).
Aircraft typically utilize gas turbine engines, which include turbine rotors that rotate to provide power. Generators are connected to each turbine engine in order to produce electrical energy, which is then utilized to power onboard electronic systems, as well as for other uses on the aircraft.
A system using a variable frequency generator and a constant frequency generator is described in U.S. patent application Ser. No. 12/406,992 to LeGros, and is hereby incorporated by reference. Typical systems constructed with the combined generator types described in the LeGros application utilize the variable frequency generator in a start mode where the generator acts as a motor and provides an initial motive force to rotate (start) the gas turbine engine. The variable frequency generator is also used in a generate mode, once the gas turbine engine has been started, to generate electrical power. The frequency of the generated electrical power varies depending on the speed of a rotor within the gas turbine engine.
The system described in the LeGros application also uses a constant frequency generator which functions only in generate mode, and is switched off during start mode. A constant frequency generator generates power having a constant frequency regardless of the rotor speed. One example constant frequency generator can be found in U.S. Pat. No. 6,838,778 to Kandil, et al., which is hereby incorporated by reference.
In order for a constant frequency generator, such as the one described in U.S. Pat. No. 6,838,778 to operate in a start mode, a torque converter, pony motor, and a two overrunning (start) clutches are included within the generator. In addition, a pony motor controller is required to power the pony motor which is within the constant frequency generator. The pony motor is used to get the constant frequency generator to synchronous speed, which then is transferred to an AC (main) bus. The torque converter is then allowed to fill with oil which allows the synchronous motor torque to provide a motive force to start the turbine engine. Many practical applications, such as an aircraft power generation system, require the minimization of the size and weight of each component. The addition of a torque converter and a pony motor runs contrary to this concept.